raft_consensus-itest: address flakiness in config-change tests
raft_consensus-itest: address flakiness in config-change tests

Several tests were waiting until all replicas had set their
LAST_RECEIVED operation to '1' and assuming that a config change could
then proceed. However, even if all servers have received index 1, this
doesn't necessarily mean that the leader has received the corresponding
acknowledgement, and thus the leader may refuse the config change.

Change-Id: I3eed4ca551a3342dcb07330c43ef94e169db7ab8
Reviewed-on: http://gerrit.cloudera.org:8080/4637
Tested-by: Kudu Jenkins
Reviewed-by: Mike Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@apache.org>

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